The present invention relates to a polyester film. More particularly, it relates to a laminated film comprising a biaxially oriented polyester film and a coat with excellent scratch resistance, antistatic properties and slip characteristics, and a polarizing plate protective film base (support), a transfer medium and a heat transfer recording medium comprising the said laminated polyester film.
Among the plastic films, polyester films are popularly used in the various fields.
Presently, hard coated films produced by applying, drying and curing hard coat on polyester films (such as polyethylene terephthalate film) having useful properties such as high heat resistance, water resistance, chemical resistance and mechanical strength are prevalently used on the windows of automobiles and buildings for the purposes of light shield, prevention of scattering of glass on accidental break of the windows, etc. Such hard coated films, however, are defective in antistatic properties and slip characteristics, and tend to collect dust and dirt due to electric charging, thus posing the problems of poor handling qualities, etc., and the improvements over such problems have been desired.
A plastic film is used for surface protection of polarizing plate.
Usually, liquid crystal display panel comprises polarizing plates laminated on both sides of a liquid crystal cell containing liquid crystal sealed between two substrates. A protective film is applied on the surface of polarizing plate for preventing scratching or collection of dust thereon in the course of distribution or in assembling process of various types of display devices in computers, word processors, TV, and such. Such a protective film is removed as a disused article after it has served for the protection of polarizing plate. Usually, removal of the protective film is accomplished by pressing a rubber type adhesive tape against the protective film and lifting it up.
As usual, polyethylene films, ethylene-vinyl acetate copolymer films and such have been used as the said type of protective film. These films, however, had disadvantages in that the film must once be separated during testing and again attached after the test because the presence of such a film may disturb the practice of the tests involving optical evaluations of display panels concerning their display performance, hue, contrast, contamination, etc.
Japanese Patent Application Laid-Open (KOKAI) No. 4-30120 proposes a protective film which need not be separated during the test involving optical evaluations, and which comprises a photoisotropic base film having a photoisotropic adhesive resin layer laminated thereon. This protective film, however, is unsatisfactory in certain respects such as chemical resistance and scratch resistance because a film formed by casting with little orientation and having a state close to amorphous is used as base film.
Plastic films are also used as a heat transfer recording medium.
The heat transfer recording media having a colorant-containing hot-melt ink layer or a heat-reactive color developing reactant layer on base paper or plastic film have been known.
In the heat transfer recording medium having a hot-melt ink layer, for example, a receptor sheet is placed thereon and thermal printing is conducted with a thermal head from the underside of the sheet, with the molten ink being transferred onto the said receptor sheet to form a color image. On the other hand, in the heat transfer recording medium having a heat-reactive color developing reactant layer, a receptor sheet or a sheet having a layer containing a substance reacted with the said color developing reactant when heated to develop color is placed on the said medium and thermal printing is conducted with a thermal head from the underside of the sheet to form a color image on the sheet surface.
In the above uses, it is required for the transfer medium to possess the antinomy properties, viz. transfer layer receptivity and transferability of the received transfer layer. The transfer medium is also required to have antistatic properties because in case where the transfer medium is electrically charged on receiving the transfer layer, the layer may not be received faithfully.
The transfer medium is further required to have anti-stick properties. Since the surface temperature of the thermal head may reach a high temperature of 300 to 400° C. or even higher, the heat transfer recording medium using a plastic film involves the problem of sticking or generation of stick remnants.
In order to avoid such a problem, it has been proposed to provide a stick preventive layer such as mentioned below on the thermal head contacting area of the plastic film. For instance, as such a stick preventive layer, there have been proposed metallic layer, heat-resistant resin layer, benzotriazole layer, ethyl cellulose layer containing sodium stearylsulfonate, and polyester resin layer containing stearic acid.
Nonetheless, any of these anti-stick layers is incapable of producing a sufficient preventive effect, and when the laminations are increased to an extent where the desired stick preventive effect can be expected, there arise such problems as reduction of heat sensitivity due to increase of heat capacity and sticking of the stick preventive layer itself or generation of stick remnants.